September is when the North Atlantic starts producing again. The first depressions of the autumn reach the island, the Reykjanes breaks wake up, and the swell arrives into a day that is still long enough to work around.
That pairing does not come back. By November the swell is at its best and the water has dropped three degrees while the daylight has halved; by March the light has returned and the water is at its coldest. September is the only month where the sea is still holding summer's 11°C and there is something to surf in it. If you want to see what this coast does without committing to a winter trip, this is when.

